1. Field of the Invention
The present invention relates to a method of operating a piezoelectric transformer for use in a power source apparatus, such as a DC-DC power source, an AC-DC power source and a DC-AC power source, and an operating circuit therefor.
2. Description of the Prior Art
A piezoelectric transformer is a voltage transforming device made of a piezoelectric material, such as piezoelectric ceramics, for example, barium titanate or lead zirconate titanate (PZT).
The basic structure of the piezoelectric transformer incorporates an operating portion provided with a primary electrode. The primary electrode applies an input voltage in a direction of the thickness of a piezoelectric vibrator composed of an elongated plate made of a piezoelectric material or a piezoelectric vibrator having a structure that elongated plates each of which is made of the piezoelectric material are stacked. Thus, the primary electrode generates mechanical vibrations. Moreover, the piezoelectric transformer incorporates a power generating portion provided with a secondary electrode which converts the mechanical vibrations into a voltage so that the voltage is taken.
The piezoelectric transformers having sizes smaller than conventional electromagnetic transformers have been widely used as high-voltage generating devices or voltage transforming devices which are employed in backlight power sources of liquid crystal apparatuses, air cleaners, ozone generators, copying machines and the like.
FIG. 6 is a perspective view showing a so-called Rosen-type piezoelectric transformer which is a typical piezoelectric transformer. Referring to FIG. 6, a Rosen-type piezoelectric transformer 10 incorporates a piezoelectric vibrator 7 having a structure formed by stacking elongated plate-like sheets each of which is made of a piezoelectric material; an operating portion 11 provided with primary electrodes 8a and 8b disposed opposite to each other in a direction of the thickness of the piezoelectric vibrator 7; and a power generating portion 12 provided with a secondary electrode 9 disposed on an end surface of the piezoelectric vibrator 7.
In the above-mentioned structure, the AC input voltage V.sub.in is applied between the primary electrodes 8a and 8b so that the operating portion 11 generates mechanical vibrations. As a result, the AC output voltage V.sub.out can be obtained at the secondary electrode 9 (where R.sub.o is load resistor). Note that a hollow arrow in FIG. 6 indicates a direction of polarization.
The piezoelectric transformer includes a resonance circuit which has a resistance component, a capacitance component and an induction component of an equivalent circuit. Therefore, the frequency characteristic of transmitting efficiency .eta. (%) of the piezoelectric transformer has peaks which are manifested at primary, secondary, tertiary, . . . , resonant frequencies f1, f2, f3, . . . .
FIG. 7 is a graph showing the relationship between frequency f (kHz) and transmitting efficiency .eta. (%) which is realized when a sine wave input voltage is applied to the Rosen-type piezoelectric transformer 10.
As shown in FIG. 7, large resonance peaks are manifested such that primary resonant oscillations (.lambda./2 mode) are manifested at 47 kHz, secondary resonant oscillations ( .lambda. mode) are manifested at 93 kHz and tertiary oscillations (3.lambda./2 mode) are manifested at 147 kHz. Thus, piezoelectric transformer 10 acts as a kind of a filter which greatly transmits electric power only at the above-mentioned resonant frequency bands.
Therefore, to obtain a large output electric power (that is, high transmitting efficiency .eta.), it is preferable that the piezoelectric transformer 10 is operated at integral multiples of 1/2 of the wavelength .lambda. of the fundamental resonant frequency in the mechanical vibrations of the piezoelectric transformer 10. The integral multiples of 1/2 of the wavelength .lambda. are the primary resonant oscillations (.lambda./2 mode), the secondary resonant oscillations (.lambda. mode), the tertiary resonant oscillations (3.lambda./2 mode), . . . . The foregoing fact has been disclosed in Text C-1-2-7 in a Symposium of Switching Power Source (sponsored by Japan Management Association), 1997.
Therefore, a single oscillation sine wave (including a waveform close to the sine wave) or a rectangular wave has usually been used by employing any one of the foregoing oscillation modes as the input AC voltage V.sub.in for operating the conventional piezoelectric transformer.
Since the piezoelectric transformer has a great input electrostatic capacity, an excessively large input electric current flows in a steep portion of the waveform if the piezoelectric transformer is operated with the rectangular wave. Thus, the piezoelectric transformer of FIG. 6 has high losses, causing heat to be generated excessively. As a result, the transformation efficiency deteriorates.
Therefore, a method has usually been employed which uses a power source circuit structured as shown in FIG. 8 so that a rectangular wave formed by a power source 1 and an operating circuit 2 (for example, a transistor switching output circuit) is brought closer to the sine waveform by an inductor L so as to be applied to a piezoelectric transformer PT. Another method has usually been employed which uses a power source circuit structured as shown in FIG. 9 so that an operating circuit 3 produces an output of a sine wave so that the piezoelectric transformer PT is operated.
Since the input voltage for the power source apparatus is determined with a rated specification, electric power which can be taken is determined with the characteristic and shape of the piezoelectric transformer.
Therefore, the size of the piezoelectric transformer must be enlarged when an attempt is made to obtain large amount of electric power by using the same input voltage in a case where input AC voltage composed of only a sine wave in one oscillation mode is used to operate the piezoelectric transformer.